1. Field of the Invention
The present invention relates to a turbine rotary blade used in a gas turbine engine, where the tip of the blade is made of a ceramic material.
2. Description of the Related Art Including Information Disclosed under 37 CFR 1.97 and 1.98
The future trend of advanced gas turbine engines for aircraft applications is to increase turbine rotor inlet temperature (RIT) to optimize turbine performance. Conventional modern turbine blades constructed of nickel-based super alloys must be substantially cooled to survive this high temperature environment. The root region of the blade must support the material above the root and is therefore subject to the highest levels of P/A stress. This region must be cooled to mitigate the effects of creep and potential stress rupture. However, the remainder of the blade, particularly the tip region, must be significantly cooled to prevent distress primarily in the form of oxidation and erosion.
Another trend of advanced gas turbines is to reduce weight by designing the turbine with the highest level of AN2 possible. This increases the weight of rotating turbine blades, posing new challenges for blade and disc structural systems. It is well known in the art of gas turbine engines that weight can be minimized by designing the turbine blades with thin walls, and airfoil taper can be utilized to provide further reductions of weight by controlling the P/A stress levels. As a result of these trends, VAATE applications require lightweight turbine structures that can withstand the rigors of extreme thermal environments.
U.S. Pat. No. 4,689,242 issued to Pike on Aug. 25, 1987 shows a turbine blade used in a gas turbine engine, the blade made of a metal with a tip member made of ceramic attached to the tip end of the blade. However, the ceramic tip of this Pike invention is of such small mass that the lightweight benefits to the rotating blade discussed above does not apply.
Other prior art airfoils like U.S. Pat. No. 4,884,820 issued to Jackson et al on Dec. 5, 1989 show an airfoil with a ceramic portion extending from the tip of the metal blade airfoil, but the ceramic tip portion does not form any part of the fluid reaction airfoil surface. The above cited Pike patent includes a very small edge portion of the ceramic tip, but this portion exposed to the fluid flow does not provide any significant surface area on which the fluid flow reacts to transfer power from the motive fluid to the rotating blade.
The composite, lightweight blade structure of the present invention addresses both concerns of increased temperature operation and lightweight structures. The present invention is based on construction of a turbine blade using a composite structure where a high temperature, lightweight material would be used in the low to moderate P/A stress field near the tip of the blade. Ceramics represent one of the possible materials for use in this region. These materials have very high temperature capability and are considerably lighter than modern super alloy materials. Use of these materials provides the potential to significantly reduce cooling requirements in the traditionally difficult-to-cool tip region. However, the properties of these materials are characteristically inferior in terms of strength and load-carrying capability, thus limiting their useful application to the relatively low-stress tip region of the turbine blade. Significant overall system weight reduction is possible with the use of lightweight materials, even if only near the tip, because weight reductions realized at the tip are propagated throughout the entire blade airfoil, attachment and rotor disc system. The root region of the airfoil would be constructed of an advanced super alloy material to provide the strength necessary in the high P/A stress region. Cooling of this region remains necessary to provide adequate life for creep and oxidation/erosion.